1. Field of the Invention
The present invention relates to a circuit arrangement for determination of a measuring capacitance.
2. Description of the Related Art
For measuring very small capacitance values or capacitance variations it has been known in the art, for example, to use capacitive proximity switches which have been known since the late sixties. They operate on the principle of influencing an alternating electric field by a dielectric of an object or a medium clearly greater than the dielectric of air or vacuum. Placing such a dielectric in the near field of one or more sensing electrodes leads to a corresponding concentration of electric field lines in that area and, accordingly, to amplification of the electric field. This has the effect to increase capacitance, for example of a plate capacitor in which the dielectric has been placed between its plates, by a small amount. That capacitance variation is evaluated in an evaluation circuit. The evaluation circuit emits a switching signal, for example, when the capacitance rises above a predefined value.
Such capacitive proximity switches are used for applications in the most diverse technical fields. Such applications include, for example, uses for monitoring limit levels of media of any kind, for example aqueous media, granulates, powders, oils and the like. They may be used in submerged condition for determining, for example, the filling level or, in a contactless way, for example by arranging them on the outside of a non-metallic wall of a container for determination of the filling level inside the container for example. Another application consists in scanning and detecting objects over a certain distance, for example in detecting a paper stack, a metallic object, a glass or the like.
Capacitive sensors are operated using electronic circuit arrangements whose functioning modes fall into two main groups, subdivided by their operating principles.
On the one hand, there are oscillating methods where as a rule the oscillating conditions of an oscillator, acting as a closed-loop amplifier, is controlled via an electrode system shortly before oscillation commences. In that case, the electrode system mostly is formed by a capacitive voltage divider, with the measuring capacitance as an integral component. The value of the measuring capacitance influences the overall amplification factor, the phase position and, thus, the oscillatory characteristics. Such oscillator circuits used for detecting small capacitance values and capacitance differences, intended for use in capacitive sensors, have been disclosed, for example, by DE 101 56 580 A1 or DE 1 673 841 A1. Such circuit arrangements and methods permit high sensitivity and/or switching intervals of the capacitive sensors and excellent “sensory qualities”, such as compensation for adhering medium residues, to be achieved with little circuitry input. A disadvantage of such oscillating methods is seen, however, in high susceptibility to interference by alternating electric fields in a wider or narrower bandwidth around the oscillating frequency. Such sensors therefore in many cases fail to meet the EMC immunity requirements, Part IEC61000-4-6.
Other methods for operating the sensors, that exist in addition to the before-mentioned oscillating methods, may be described as “driven ” methods where a generator controls a measuring circuit according to different principles, for example as capacitive bridges, oscillating amplifiers, charge-balancing methods, phase comparators, or the like.
Capacitive sensors that operate on that principle have been disclosed, for example, by DE 199 49 985 A1, DE 197 01 899 C2, EP 1 093 225 B1 or by DE 10 2005 057 558 A1. Although in many cases capacitive sensors that are operated in this way are less sensitive to inference voltage in a wide frequency spectrum it is, however, a disadvantage that such sensors can be realized only with considerably higher circuitry input and in most cases with poorer sensory quality. As used in the present application, the term “sensory quality” relates to the following values or properties:                a large adjustable and usable sensitive range, i.e. a large span between the minimally and the maximally adjustable switching interval;        little temperature dependence of the switching interval;        the capability to compensate for wet, conductive adhering media, in the case of filling level applications;        series stability, or independence from component variations.        
Presently, there do not exist any capacitive sensors that provide those properties in combination with high EMC immunity. For example, the sensor for contactless detection of the filling level of a highly conductive liquid and adhering medium, such as blood, through a non-metallic container wall of a container, as described by DE 10 2005 057 558 A1, and the method disclosed by that publication actually provide for excellent compensation for adhering media in filling level applications, combined with high interference immunity. But on the other hand, that sensor and the method for contactless detection of the filling level of liquid media do not reach the high switching intervals required in distance sensor technology.
While the method for detection and evaluation of a capacitance variation disclosed by DE 199 45 330 A1, and the sensor described in that publication, permit, for example, high switching intervals to be achieved and high interference levels to be compensated, that sensor does not provide for compensation for adhering ionizing media.